add .bazelversion (#6990)

## Motivation for features / changes

add .bazelversion to make bazelisk happy

This is very helpful for people who work in several different bazel
workspaces.

## Technical description of changes

## Screenshots of UI changes (or N/A)

N/A

## Detailed steps to verify changes work correctly (as executed by you)

## Alternate designs / implementations considered (or N/A)

N/A

Author: hunshcn

.bazelversion

@@ -0,0 +1 @@
+6.5.0